The present invention relates to high phase order electrical rotating machines.
When the drive motor is used as a brake during operation of an elevator with the induction motor driven by an inverter, the rotating speed of the motor is higher than the frequency of the inverter, and regenerated power is formed in the motor. As this regenerated power flows into the DC circuit of the inverter, a resistor in the DC circuit absorbs the regenerated power. A number of approaches for controlling this have been disclosed; for example Kitaoka and Watanabe (U.S. Pat. No. 4,678,063) disclose an elevator control system connected to a source of three-phase alternating current which is rectified by a converter to direct current which is converted to a variable-voltage variable-frequency A.C. voltage which, in turn, drives the elevator hoist motor. A resistor and a switch are connected across the D.C. terminals of the converter. When the motor is operating in the regenerative mode, the switch is closed to permit the regenerated circuit to flow through the resistor, which dissipates or consumes the regenerated power. When the regenerated power being consumed by the resistor is detected to exceed a predetermined value, the excess regenerated power is returned to the A.C. source through a regenerative inverter. Kanzaki and Yamada (U.S. Pat. No. 5,420,491) disclose the conversion of an analog DC link bus voltage in an induction motor drive to a digital DC link bus voltage so that if the DC link bus voltage exceeds an ON voltage threshold of a switch and this condition exists a holding time later, the switch closes, thereby allowing regenerated power in the DC link to be dissipated in a resistor connected across the DC link. And if the DC link bus voltage falls below an OFF voltage threshold and that condition exists a latching time later, the switch is opened so that no regenerated power may be dissipated through the resistor.
Systems disclosed in the prior art use a variety of methods for sensing when regenerated power on the DC supply line, but they all use a switch (a transistor in the case of Kitaoka and Watanabe) to allow the regenerated power to flow through the resistor.